On the influence of several meteorological factors on polarization state of microwave signals, propagating in semi-arid terrestrial links

The cause for the changes in polarization state of microwave signals is searched after physical properties of the channel of propagation and their variations in accordance with atmospheric state. The characteristics of the air, as propagation medium, are determined by the content, concentration and physical properties of the suspended particulates; especially it is noticeable in the surface layer over the areas with high levels of aerosol concentration and humidity (urban, semi-arid, highways, etc.). The interaction between aerial water and solid particulates leads to the appearance of increased amounts of enlarged grains with changed dielectric permittivity ε˜_g, which value depends on the water fraction in the grain, particularly in the centimeter wave range. The formation of less or more dense volumes with various ε˜_g entails displacement of the transmitted signal, while wind exerts control over the distribution of such irregularities in the air. Fairly high correlation (with coefficients |ρ|>0.45) between temperature, humidity, concentration of suspended particles and winds, and the power P_r and cross-polarization discrimination (XPD) characteristics (XPD and δ) of microwave signals was observed under moderate weather conditions. During a quiet 24-h cycle the mean level of P_r and XPD changes in ∼ 10% and 30%, correspondently; fastest variations in depolarization degree are obtained close to sunset and after sunrise, when the large gradients of meteorological parameters and maximum instability of air streams take place. Seasonal fluctuations of P_r and XPD are registered, too.